We had a very bad experience with a vet and don't really trust her when she says that nothing we can do can help our older cat develop more muscles. She says once it's gone, it's gone for good. How can that even be true when muscles are one of the most regenerative tissues in the body? Would a high protein diet help him? What about supplements and exercise?

His blood work was clean. No kidney problems.

He is an older (12+) tuxedo Manx Coon. The Vet prescribed Hill's Z/D hydrolyzed protein canned food or dry for both his nutritional upkeep and hairball treatment. However, this has less protein than the Fancy Feast we had been giving him.